I wouldn't mind as much, but lately there hasn't been much progression in the lore. Thresh comes in looking for fresh people to torture and..........nothing more happens. Does he get to torture people outside of the League? Does he capture Amumu and try to drive him insane? What does the League think of his torturing? Do they just let anyone join these days, even if you are feeding people to a spider or torturing them?

IMO either the champs from the Shadow Isles are joining the League to try to find weaknesses to exploit for some later attack, or they're being captured by the League so they can't kill/torture more people, like Brand. Who knows - maybe Thresh is in the League because he DID try to attack a champion outside the League, and the champion escaped and alerted the summoners. In any case, Thresh probably wouldn't mind a chance to torture the souls of the biggest and baddest.

And really until we know more about the desires of the Shadow Isles we can't make assumptions so readily. We do know they have hidden agendas and serve a higher purpose. They are all devious and labyrinthine in their motives.

He has a reason to join, a step in the right direction for the storytelling, but the reasons itself are shaky and flimsy at least.

He joins to steal souls away to the Shadow Isles:
That'll be hard considering that the League won't let him do that out the Fields. Doing it in the Fields won't amount to anything.

He wants to break the sanity and spirit of the champions
Somewhat more sensible than the previous, but not by much. A champion has to endure being shot, burned, devoured by sharks/trolls/void creatures, cut, incinerated, mauled, blown up, poisoned, subject to fear/nightmares/visions of their demise, turned to stone, hit by infected cleavers, decapitated etc... every time they're summoned.
And that was before Thresh came out. What's a guy with a chain and spooky powers got that makes all the others previously mentioned lesser in comparison?

The end problem with all characters is that their stories don't progress as they have in the past, preventing us to see what they are doing once they are in the League.

In this way, all lore is well... Not in a good state. It prevents us from fully understanding how characters act outside the League. Thresh falls under that category. I suppose we'll see what happens next I suppose in 2013.

Well if we had the JoJ we would had some news about it, stating how two spectral spirits was last reported at Demacia with dead bodies galore.